Photolithographic patterning is a well-established technology in the manufacturing processes of various kinds of semiconductor devices and liquid crystal display panels. According to photolithographic patterning, a photosensitive resist composition is first coated onto a surface of a substrate to form a photoresist layer. The photoresist layer is then exposed to radiation, such as ultraviolet light or electron beam, so that some portions of the photoresist are impacted by radiation while other portions of the photoresist are not impacted by the radiation. Subsequently, the photoresist is subjected to a developer solution, which selectively removes either the impacted or non-impacted portions of the photoresist. If the photoresist is a positive photoresist, the impacted portions are selectively removed; if the photoresist is a negative photoresist, the non-impacted portions are selectively removed. The photoresist material remaining after development shields or masks the regions of the substrate from subsequent etch or implant operations.
In recent years, the minimum feature size of advanced ULSIs has reached the resolution limits of the conventional optical lithography technology. For example, it is known that i-line lithography technology is adequate for forming a contact hole having a minimum feature size of over 0.30 to 0.35 μm. Thus, with current i-line optical lithography systems, the minimum feature size below sub-0.18 μm would be very difficult to achieve.
Various attempts to solve the resolution problem, particularly the extension of the optical lithography technology for sub-0.18 μm contact hole lithography, have been known in the art. These attempts include (i) phase shift mask (PSM) technology; (ii) off-axis illumination (OAI) technology; (iii) resist reflow technology; and (iv) a multi-layer resist (MLR) processing. Among these technologies, the resist reflow technology is most desirable because of its simplicity.
According to conventional reflow processes, the photoresist layer is typically applied as a thick photoresist layer, with a thickness over 0.5 microns. The thick photoresist layer is patterned and the photoresist layer is subsequently developed and heated to a high temperature (e.g., between 120 to 170° C.). During heating, the photoresist layer becomes almost plasticized (e.g., viscous or semi-liquid) and the photoresist material flows due to the high temperatures associated with the heating step. The heating of the thick photoresist layer reduces the width associated with features in the resist pattern because the edges of the resist pattern flow closer together, therefore making a smaller hole or trench. After the photoresist layer has been heated (i.e., reflowed), conventional semiconductor processes are conducted.
Reflow technologies require thick photoresist layers to ensure that a sufficient amount of material is available to reflow. However, the use of thick photoresist layers has an adverse effect on lithographic resolution. Variations in thickness uniformity can affect the precision associated with focusing the radiation on the photoresist layer (i.e., it is difficult to have a precise depth of focus when the photoresist layer is thick). Other conventional processes have utilized ultrathin photoresist layers. Ultrathin photoresist layers have achieved greater resolution than thick photoresist layers. However, reflow technologies have not been applied to ultrathin photoresist layers because the ultrathin photoresist layer does not provide adequate material for the flow operation (i.e., the ultrathin photoresist layer is too thin to provide sufficient material to flow without compromising other areas of the photoresist layer). Another disadvantage of both the thick and ultrathin photoresist layers is that, although the area of the aperture (hole or trench) can be reduced, the critical dimension (CD) control is very difficult in the reflow technology because severe overhang often results in the photoresist contact hole pattern. In addition, the photoresist contact hole pattern could collapse or close at high temperatures.
Accordingly, there is a need for a reflow stabilizing solution that acts as a stopper control flow and allows precise CD control in reflow techniques for both thick and ultrathin photoresists. Also needed is a controlled resist reflow process for reducing the dimensions of resist apertures, while preventing overhang or collapse of the resist patterns. A method of forming a pattern resist mask for fabricating structures having line width dimensions in the submicron range is also needed.